Digital interferometers are being increasingly used in a number of production processes to characterize and measure surfaces. Optical measuring devices of this kind are distinguished by a high degree of measuring accuracy. For manufacturing, it is desirable to make measurements without contacting the measured part to avoid damage. It is also desirable that measurements be fully automated.
A measuring device of this kind is the Laser Spot Scanning Interferometer (LASSI) described in U.S. Pat. No. 4,298,283 assigned to the assignee of the present invention, incorporated herein by reference. The underlying measuring principle is based on scanning the surface to be tested with two laser light beams which are simultaneously focused adjacent to each other onto the surface. During this process, the optical phase difference between the two light waves, which are reflected from the surface, changes linearly as a function of the height difference between the two laser spots on the surface. The phase difference is determined by phase switching. For this purpose, an electro-optical light modulator is used which periodically shifts the phase difference between the two light waves by a fixed amount. At the same time, the intensity of the two interfering light beams is measured by a photodiode. With P.sub.S, P.sub.R and P.sub.T being the light powers measured for the phase shift values .phi.=0,.+-.2/3, the phase difference to be measured ##EQU1## where
.lambda. is the light wavelength and
h is the height difference,
is determined according to the following equation: ##EQU2##
The phase measuring value .phi..sub.M is calculated according to that equation, using, for example, a personal computer controller.
The above described phase measuring method provides the exact phase measuring value .phi..sub.M and thus the height difference value h to be determined, provided that the modulation voltage at the modulator shifts the phase periodically by .+-.2/3.pi.. However, in practice, this switching voltage is not constant. Similar to the half-wave voltage of the modulator, the switching voltage, depending upon the ambient temperature, is subject to slight variations which may exceed 5% of the rated voltage. To reduce errors resulting therefrom during phase measurement to a minimum, the half-wave voltage frequently has to be redefined by an automatic calibration routine. Another error, attributable to the sequential processing of the measuring value, occurs when a rapidly changing phase measuring value is dynamically determined. In a phase measuring process, the measuring time of the three light power values P.sub.S, P.sub.R and P.sub.T is several milliseconds. During this time, the phase value .phi..sub.M to be measured must remain unchanged. For.scanning profile height measurements, this may be achieved by a correspondingly slow lateral movement of the measuring object. However, in the case of unforeseeable rapid phase changes, such errors and their effect cannot be avoided.